As the technology development and demand for mobile devices increase, the demand for secondary batteries as an energy source is rapidly increasing. Among such secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle life, and low self-discharging rate have been commercialized and widely used.
As positive electrode active materials of lithium secondary batteries, lithium transition metal composite oxides have been used. Among these oxides, a lithium cobalt composite metal oxide of LiCoO2 having a high operating voltage and excellent capacity properties has been mainly used. However, since LiCoO2 has very poor thermal properties due to an unstable crystal structure caused by lithium deintercalation, and also is expensive, there is a limitation in using a large amount of LiCoO2 as a power source for applications such as electric vehicles.
As materials for replacing LiCoO2, a lithium manganese composite oxide (LiMnO2 or LiMn2O4), a lithium iron phosphate compound (LiFePO4, etc.), and a lithium nickel composite oxide (LiNiO2, etc.) have been developed. The research and development of a lithium nickel composite oxide among these materials which has a high reversible capacity of about 200 mAh/g, thereby allowing for a bigger capacity battery to be easily implemented, has been more actively conducted. However, when compared with LiCoO2, LiNiO2 has limitations in that the thermal stability thereof is poor, and when an internal short circuit occurs in a charged state due to pressure from the outside and the like, a positive electrode active material itself is decomposed causing the rupture and ignition of the battery.
Accordingly, as a method for improving the low thermal stability while maintaining the excellent reversible capacity of LiNiO2a, a method of substituting a portion of nickel (Ni) with cobalt (Co) or manganese (Mn) has been proposed. However, in the case of LiNi1-αCoαO2 (α=0.1˜0.3) in which a portion of nickel is substituted with cobalt, there are limitations in that the excellent charge and discharge properties and lifespan properties were obtained, but the thermal stability was low. Also, in the cases of a nickel manganese-based lithium composite metal oxide in which a portion of Ni is substituted with Mn having excellent thermal stability, and a nickel manganese cobalt manganese-based lithium composite metal oxide (hereinafter, simply referred to as a “NMC-based lithium oxide”) in which a portion of Ni is substituted with Mn and Co, the cycle properties and thermal stability are relatively excellent. However, due to the low resistance thereof, when a metal body such as a nail penetrates, an internal short circuit does not occur, causing serious problems in terms of safety such as ignition or explosion due to instantaneous overcurrent.